This invention relates to a liquid component measuring apparatus for measuring a specific component in a liquid sample.
As disclosed in Japanese Unexamined Patent Publication (JP-A) No. 3-273153 (273153/1991), a conventional liquid component measuring apparatus of the type is used in a clinical test in successively and easily measuring a specific component of, for example, glucose or urea contained in a liquid sample of, for example, body fluid with high accuracy. The body fluid is exudation fluid obtained by removing blood and horny layers from skin and by subjecting the skin to a suction processing under a reduced pressure so that the exudation fluid is exuded from the skin.
Referring to FIG. 1, the conventional liquid component measuring apparatus comprises a housing 31 and a cell 34 coupled thereto. The housing 31 is provided with a sampling port 37, a suction port 38, and a diluting liquid introducing port 39. Within the housing 31, a valve 32 having a through hole 33 is arranged to be rotatable. When the valve 32 is rotated to a position illustrated in the figure, the through hole 33 is aligned with the sampling port 37 and the suction port 38 to define a sampling path or channel for the liquid sample. The through hole 33 also serves as a liquid sample storage chamber storing a predetermined amount of the liquid sample. On the other hand, when the valve 32 is rotated to another position, the through hole 33 communicates with the diluting liquid introducing port 39 and an internal cavity of the cell 34. The cell 34 is provided inside with a stirrer 17 and a sensor 15 and has a discharge port 40.
Referring to FIGS. 2A through 2D, operation of the conventional liquid component measuring apparatus will be described in conjunction with measurement of glucose concentration in the body fluid exuded as the exudation fluid. For clarity of illustration, only the body fluid, the diluting liquid, and the body fluid after diluted are shown as hatched portions.
At first referring to FIG. 2A, the valve 32 (FIG. 1) is kept at an initial position where the through hole 33 extends in a horizontal direction and communicates with the diluting liquid introducing port 39 and the interior of the cell 34. The diluting liquid depicted at 41, for example, an HEPES buffer solution of 20 mM and pH 7.5 is introduced through the diluting liquid introducing port 39 to fill the through hole 33 of the valve 32 and the interior of the cell 34.
Next referring to FIG. 2B, the valve 32 is rotated by 90.degree. to align the through hole 33 with the sampling port 37 and the suction port 38 so that the sampling path is formed. Then, the through hole 33 is filled with the body fluid 42.
Turning to FIG. 2C, the valve 32 is rotated by 90.degree. to return to the initial position where the through hole 33 communicates with the diluting liquid introducing port 39. Then, the diluting liquid 41 is fed through the diluting liquid introducing port 39 to extrude the body fluid 42 filled in the through hole 33 to the interior of the cell 34. Within the cell 34, the body fluid 42 is stirred by the stirrer 17 and diluted by the diluting liquid 41, as illustrated in FIG. 2D. The glucose concentration is measured by a glucose sensor arranged in the cell 34.
At this time, a dilution ratio of the body fluid is exactly determined by the ratio between the volume of the through hole 33 of the valve 32 and the effective volume of the cell 34 so that the dilution is carried out with high accuracy. Generally, the body fluid does not have a constant pH or a constant pH buffer capacity. However, by the use of a pH buffer solution constant in pH, pH buffer capacity, and ionic strength and sufficient in oxygen/enzyme concentration as the diluting liquid, it is possible to control the pH, the pH buffer capacity, the ionic strength, and the oxygen/enzyme concentration of the liquid sample to be measured. Thus, the measurement is carried out with high accuracy.
In order to calibrate the sensor, the cell 34 may further be provided with another inlet port for a reference liquid having a known component. The above-mentioned apparatus can be applied not only to the exuded fluid and the blood but also to various other liquids. The sensor may be selected from various types in correspondence to the liquid sample to be measured.
Another conventional liquid component measuring apparatus of the type is disclosed in U.S. Pat. No. 5,429,726. This apparatus is used to successively and easily measure the amount of L-Asp-L-Phe-methyl ester in soft drinks with high accuracy.
Referring to FIG. 3, the liquid component measuring apparatus disclosed in the above-mentioned United States patent comprises a polarographic cell 100 including an electrically insulating container 54 composed of a suitable dielectric material, such as glass or plastic. The container 54 is covered by an electrically insulating cap 56. The cap 56 is provided with an aperture 58 through which an electrolyte 60 is admitted to the cell 100. An electrically insulating rod or column 62 extending from the cap 56 downwardly into the cell 100 is provided with a conductor 64. The conductor 64 is connected at the distal end thereof to a working or sensor electrode 66 which may be composed of platinum, gold, silver, graphite, or the like. The proximal end of the conductor 64 is connected with a DC voltage source 68.
A reference electrode 70 is provided between the column 62 and the walls of the container 54. The reference electrode 70 may comprise a silver chloride coated silver wire. The electrolyte 60 fills the space between the reference electrode 70 and the working electrode 66.
The lower end of the container 54 is provided with an O-ring 74 snap fit into an annular groove 76 on the outer wall of the container 54 to hold a laminated enzyme-containing membrane 78 securely in fluid tight relation over the bottom of the container 54.
Surrounding the bottom portion of the polarographic cell 100 is a reservoir 200 adapted to contain an analyte containing solution or buffer. The reservoir 200 includes an analyte injection channel 202 and a buffer injection channel 204. An overflow weir 206 provides for overflow to drain from the apparatus.
The first-mentioned conventional apparatus requires a high-accuracy metering pump as the feeding pump and the valve for switching a fluid path or channel alternately to feed the diluting liquid and to introduce the liquid sample. Therefore, the apparatus inevitably becomes bulky in size. This results in restriction in portability and measuring site.
In addition, the housing is provided with the valve, a motor for driving the valve, the through hole, the sampling port, the diluting liquid introducing port, and the suction port while the cell is provided with the sensor, the stirrer, and the discharge port. Thus, the first-mentioned apparatus requires a number of components and is therefore complicated in structure.
In the second-mentioned conventional apparatus, the sensor is incorporated into an electrochemical reactor chamber. In order to exchange the sensor, at least a part of the electrochemical reactor chamber must be disassembled. Thus, exchange of the sensor is troublesome and inconvenient.
The liquid sample to be measured by the apparatus often contains foreign particles or impurities, for example, coagulated and deposited protein and bubbles. In this event, the dilution ratio may become inaccurate and the interior of the cell can not sufficiently be cleaned. In this case, the accuracy of measurement will be degraded for some types of the samples.
In addition, since the liquid component measuring apparatus comprises a number of components and is therefore complicated in structure as described above, a production cost will be increased owing to such complicated structure.